Hexavalent chromium is used in many industrial fields such as electroplating, production of coating materials, tanning and the like. Wastes generated from such industries is liable to leak from a storage container or piping and percolate into underground. Hexavalent chromium is a substance acting on a human body as a powerful carcinogen and dissolves in water in the form of a chromate or a bichromate to contaminate groundwater or soil, causing a serious environmental problem.
As a technique for remediating soil or groundwater contaminated with hexavalent chromium, dig and dispose, solidification or immobilization, or containment is generally used. As an in-situ decontamination technique, there have been disclosed a method which comprises injection of an oxidizing agent such as a permanganate, ozone, hypochlorous acid, hydrogen peroxide or the like (see, for example, Patent Document 1), a method which comprises treating soil with an aqueous citric or tartaric acid solution to remove a hazardous heavy metal (see, for example, Patent Document 2), and phytoremediation, in which plants absorb and accumulate heavy metals to remove the contaminants from soil (see, for example, Patent Document 3).
Further, a technique which comprises reducing hexavalent chromium to trivalent chromium that is water-insoluble to effect decontamination is potential one because it is capable of decontaminating in situ and can be carried out at a low cost. In this method, when hexavalent chromium is once reduced to trivalent chromium (chromium (III) hydroxide), the resultant is filtered by soil particles and thereby removed from groundwater.
Various chemical substances and microbial biostimulants are used in reduction of hexavalent chromium in soil or groundwater. Of these, zerovalent iron (hereinafter referred to as iron (0)) is one of substances which are most highly effective in reducing hexavalent chromium to trivalent chromium and used in various applications in the field of hexavalent chromium-decontamination as a reducing agent.
In Patent Document 4, there is described a method which comprises forming a dispersed layer of powdery iron by applying iron powder into the ground to decontaminate groundwater passing therethrough. Patent Document 5 discloses a method which utilizes iron as a reducing agent in a permeable reactive barrier to reduce hexavalent chromium contained in groundwater passing through the reaction wall to trivalent chromium. In Patent Document 6, there is described a decontaminant containing iron which is mixed with a contaminated medium to reduce hexavalent chromium. Each of Patent Documents 7 and 8 discloses a method which comprises injecting ferrous sulfate into a contaminated medium or mixing ferrous sulfate with a contaminated medium to convert hexavalent chromium to trivalent chromium through chemical reduction reaction, thereby effecting decontamination.
When ferrous iron is used as a reducing agent, the following reactions occur between ferrous iron and hexavalent chromium. According to the following formula 2, 3 mols of ferrous iron are required to reduce 1 mol of hexavalent chromium.Cr2O72−+6Fe2++14H+→2Cr3++6Fe3++7H2O  formula 1CrO42−+3Fe2++14H2O→Cr3++3Fe3++8OH−  formula 2
In the nature, microorganisms capable of reducing hexavalent chromium in aerobic or anaerobic conditions exist (see, for example, Non-Patent Documents 1 and 2). Some of these microorganisms utilize hexavalent chromium as a terminal electron acceptor to obtain energy. As a heavy metal-decontamination technique utilizing such microorganisms, there is described a method which comprises supplying carbon and energy to indigenous hexavalent chromium reducing microorganisms by adding blackstrap molasses or cooking oil as a decontaminant to utilize biological reduction reaction in hexavalent chromium-decontamination in each of Patent Documents 9, 10 and 11.
Patent Document 1: Japanese Unexamined Patent Publication No. 2002-096057
Patent Document 2: Japanese Unexamined Patent Publication No. Hei 11-156338 (1999-156338)
Patent Document 3: Japanese Unexamined Patent Publication No. 2002-336837
Patent Document 4: Japanese Unexamined Patent Publication No. 2002-200478
Patent Document 5: U.S. Patent Application Publication No. 2001/0054588
Patent Document 6: Specification of U.S. Pat. No. 5,975,798
Patent Document 7: Japanese Unexamined Patent Publication No. 2001-121131
Patent Document 8: Japanese Unexamined Patent Publication No. 2000-202421
Patent Document 9: Specification of U.S. Pat. No. 6,143,177
Patent Document 10: Specification of U.S. Pat. No. 6,322,700
Patent Document 11: Specification of U.S. Pat. No. 6,398,960
Non-Patent Document 1: “Aerobic reduction of hexavalent chromium in soil by indigenous Microorganisms” (Bader et al.), Bioremediation Journal, 1999, No. 3, pp. 201-211
Non-Patent Document 2: “Factors affecting chromate reduction in Enterobactor cloacae strain H01” (Komori et al.), Applied Microbiology and Biotechnology, 1989, No. 31, pp. 567-570